Arum played down earlier reports that Rios tested positive for steroids, saying that the substance probably came from a dietary supplement the American brawler used in order to make weight for his fight against Pacquiao.

The 27-year-old Rios fought Pacquiao at a catch weight of 147 lbs, but he reportedly struggled to make weight in his first fight as a welterweight despite coming from the lower 140-lb division.

"There was a little something in his urine after the fight," Arum told Boxingscene. "It was something the VADA people reported to the Chinese commission after the fight. He passed all the tests before that. It wasn't a steroid. It was probably something he took to make weight."

Alex Ariza, who worked as the strength and conditioning coach of Rios, came under fire with some experts blaming him for the failed drug test, but he quickly denied the allegations. Ariza reiterated that Rios did not test positive for a performance enhancing drug.

"Again, it's not a performance enhancing drug. It's not any of those illegal, class 3 things. If it's something that you can buy over the counter, maybe, I don't know. Again, I don't know how something like that - in China, maybe he drank something, it could've been any of those things. Again, it's a dietary supplement. People have been coming up and telling me, it is FDA approved, like it's not a banned thing or anything like that. I don't know the details. I don't really know," Ariza said in a Boxingscene interview.

Ariza also questioned the testing procedure done by VADA after the fight, revealing that the equipment used by the testing agency could have affected Rios' post-fight test results.

"We think that there might have been something - at the end of the fight, the containers VADA used to retrieve the urine samples weren't available, so they had Brandon pee into a glass," Ariza said. We have to see why he had to pee in a glass and then it was transferred from a regular drinking glass into a thing. There's a lot of contaminants. I'm not saying that they did that. It's just kind of odd that we went through four testings and we never tested positive for anything."

Rios will likely face suspension as a consequence of failing the post-fight drug test if his B-sample also tests positive for the same substance.